The present invention relates to the manufacture of hollow articles such as bottles, drums or barrels, from a thermoplastic by the technique of blow-molding of preforms.
Numerous techniques for manufacturing hollow bodies are already known, according to which preforms are produced, in a first stage, from a thermoplastic and these preforms are then transferred into final blow-molds in order to produce the desired hollow articles.
These techniques permit the production of hollow articles of a thermoplastic which is molecularly oriented. These hollow articles thus possess greatly improved mechanical properties. To manufacture these articles, an additional stage of heat conditioning of the preforms is generally provided before the preforms are introduced into the final blow-molds. In these techniques, preforms with a closed bottom are generally employed.
The processes and apparatuses currently known however suffer from certain important disadvantages which stem from the need to transfer the preforms.
Thus, if the preforms having a closed bottom are produced by blowing a tubular parison, they possess a weld line in the region of their bottom, which is caused by the flattening of the parison during closing of the molds used to produce them. The preforms then undergo a series of transfers before they are introduced into the final blow-molds. As a result, the weld line of the bottom of each preform can have any randomly occurring angular position at the instant when the preform is introduced into its associated final blow-mold. One consequence of this is that it is not possible to reliably utilize the final blowing operation in order to imprint relief inscriptions onto the bottom of the hollow bodies produced, because there is the danger that these inscriptions may be rendered illegible by the weld line of the bottom.
Furthermore, if it is desired to produce by this process hollow bodies having a highly elaborate neck, it is advantageous to mold this neck during the production of the preforms because the thermoplastic is then at a high temperature which renders molding easier and more precise. However, in practice this is difficult to do because there is a danger that the necks molded on the preforms will become damaged during the subsequent trapping of the preforms in the final blow-molds. The use of the technique is impossible if the necks are not axially symmetrical, such as necks provided with a thread.